System, Device and Method for Distributing Link State Information in a Communication Network

ABSTRACT

A system, device, and method for distributing link state information in a communication network combines a link state routing protocol with a sliding window mechanism in order to efficiently distribute link state information. The sliding window mechanism permits a predetermined number of unacknowledged link state advertisement protocol messages to be outstanding at any given time. Unacknowledged link state advertisement protocol messages are retransmitted after a predetermined timeout period.

FIELD OF THE INVENTION

The present invention relates generally to communication systems, andmore particularly to distributing link state information in acommunication system.

BACKGROUND OF THE INVENTION

In today's information age, communication networks are often used forinterconnecting computers and computer peripherals. A communicationnetwork typically includes a number of nodes that interoperate to routeprotocol messages. The various nodes in the communication networkutilize various routing protocols in order to determine the routes thatare used to route the protocol messages.

One type of routing protocol, known as a “link state” routing protocol,determines routes based upon the status of communication links betweenthe various nodes. A link state routing protocol, such as the OpenShortest Path First (OSPF) routing protocol, requires each node to havecomplete topology information. Each node maintains a topology databasethat indicates all nodes in the communication network and lists thecommunication links that are associated with each node.

The various nodes in the communication network exchange link stateinformation using link state advertisement (LSA) protocol messages. Linkstate information is exchanged at various times. In particular, when anode is initialized, the node needs to obtain link state information inorder to determine routes for routing protocol messages, and thereforethe node's neighbors send LSA protocol messages to the node in order toprovide the node with the necessary link state information. Also, eachnode periodically tests the communication links to each of its neighborsand sends a LSA protocol message including the link status informationto all of the other nodes. When a failure occurs in the communicationnetwork (such as a communication link failure or a node failure), thevarious nodes in the communication network need to obtain updated linkstate information in order to determine new routes for routing protocolmessages around the failure, and therefore the nodes adjacent to thefailure send LSA protocol messages to the other nodes in thecommunication network in order to inform all nodes of the failure. Eachnode computes the routes based upon the link status information receivedfrom the other nodes.

Any time link state information is exchanged, it is desirable for thelink state information to be distributed quickly in order for thereceiving node(s) to determine new routes for routing protocol messages.Unfortunately, routing protocols utilize a “stop-and-wait” mechanism fordistributing link state information. When a node sends an LSA protocolmessage to a neighbor, the node waits for an acknowledgment from theneighbor before sending another LSA protocol message. This is a verysimple but inefficient way to distribute link state information.

An efficient technique for distributing link state information isneeded.

SUMMARY OF THE INVENTION

In accordance with one aspect of the invention, a link state routingprotocol utilizes a sliding window mechanism to efficiently distributelink state information. The sliding window mechanism permits apredetermined number of unacknowledged link state advertisement protocolmessages to be outstanding at any given time. Unacknowledged link stateadvertisement protocol messages are retransmitted after a predeterminedtimeout period.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and other objects and advantages of the invention will beappreciated more fully from the following further description thereofwith reference to the accompanying drawings wherein:

FIG. 1 is a block diagram showing an exemplary communication network inaccordance with an embodiment of the invention;

FIG. 2 is a block diagram depicting a series of LSA protocol messages inaccordance with an embodiment of the invention;

FIG. 3 is a message flow diagram depicting an LSA protocol messageexchange by a link state routing protocol using a “stop-and-wait”mechanism as known in the art;

FIG. 4 is a message flow diagram depicting an LSA protocol messageexchange by a link state routing protocol using a sliding windowmechanism in accordance with an embodiment of the invention;

FIG. 5 is a block diagram depicting a sliding window in a first positionduring an LSA protocol message exchange in accordance with an embodimentof the invention;

FIG. 6 is a block diagram depicting a sliding window in a secondposition during an LSA protocol message exchange in accordance with anembodiment of the invention; and

FIG. 7 is a block diagram depicting a sliding window in a third positionduring an LSA protocol message exchange in accordance with an embodimentof the invention.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT

An exemplary embodiment of the present invention uses a sliding windowmechanism for distributing link state information. A sliding windowmechanism permits a node to send multiple protocol messages withoutwaiting for individual acknowledgments. The node can therefore havemultiple unacknowledged protocol messages outstanding at any given time.The number of unacknowledged protocol messages that are permitted to beoutstanding is referred to as a “window size.” A typical embodimentmight use a window size of eight (8) or 128, although the window sizemay be set to other values and may be set based upon networkcharacteristics (such as round trip delay to a neighbor) or otherconsiderations. It should be noted that a window size of one (1)degenerates to a “stop-and-wait” mechanism. The neighbor may acknowledgeprotocol messages one at a time, or may acknowledge multiple protocolmessages together. Unacknowledged protocol messages may be retransmittedafter a predetermined timeout period.

In an exemplary embodiment of the present invention, a node is permittedto have a predetermined number of unacknowledged LSA protocol messagesoutstanding at any time. Thus, when the node needs to send link stateinformation to a neighbor, the node is permitted to send multiple LSAprotocol messages without waiting for individual acknowledgments foreach LSA protocol message. The window size may be sufficient for thenode to send all LSA protocol messages that it needs to send to theneighbor. However, assuming the node needs to send more LSA protocolmessages than the window size allows, the node stops sending LSAprotocol messages if and when the maximum number of unacknowledgedoutstanding LSA protocol messages is reached. The window size ispreferably set so that, as long as there are no communication errors,the neighbor will acknowledge the LSA protocol messages quickly enoughthat the node does not have to stop sending LSA protocol messages. If acommunication error occurs, the node may retransmit any unacknowledgedLSA protocol messages.

There are many types of sliding window mechanisms, and an embodiment ofthe present invention may employ any applicable sliding windowmechanism. The present invention is in no way limited to the type ofsliding window mechanism employed.

In an exemplary sliding window mechanism, the node inserts a sequencenumber in each LSA protocol message. The sequence numbers typicallystart at zero (0) and increment to the window size minus one (1) beforewrapping back to zero (0). In other words, the sequence number is moduloN, where N is the window size. The neighbor may be required toacknowledge each individual LSA protocol message, for example, bysending the sequence number of the LSA protocol message beingacknowledged, or the neighbor may be permitted to acknowledge multipleconsecutive LSA protocol messages, for example, by sending the sequencenumber of the last LSA protocol message in the sequence of LSA protocolmessages being acknowledged. The neighbor may or may not be permitted toacknowledge LSA protocol messages received out of sequence or receivedafter a dropped or missing LSA protocol message. Various retransmissiontechniques require the node to save unacknowledged LSA protocol messagesin case the node needs to retransmit one or more LSA protocol messages.The node may be required to retransmit all LSA protocol messagesfollowing an unacknowledged LSA protocol message, or may be permitted toretransmit individual unacknowledged LSA protocol messages.

FIG. 1 is a block diagram showing an exemplary communication network 100in accordance with an embodiment of the invention. The communicationnetwork 100 includes, among other things, a node 102 and a neighbor 106coupled over a communication link 104. The node 102 and the neighbor 106interoperate to route protocol messages within the communication network100. Specifically, the node 102 and the neighbor 106 implement a linkstate routing protocol for determining routes within the communicationnetwork 100.

Implementation of the link state routing protocol requires the node 102and the neighbor 106 to exchange LSA protocol messages over thecommunication link 104. During a particular LSA protocol messageexchange, the node 102 may need to send multiple LSA protocol messagesto the neighbor 106 over the communication link 104. FIG. 2 depicts aseries of eleven (11) protocol messages 200 to be sent by the node 102to the neighbor 106 over the communication link 104.

In a prior art embodiment, the node 102 sends the series of LSA protocolmessages 200 to the neighbor 106 over the communication link 104 using a“stop-and-wait” mechanism. FIG. 3 is a message flow diagram depicting anLSA protocol message exchange by a link state routing protocol using a“stop-and-wait” mechanism. The node 102 sends one LSA protocol messageat a time to the neighbor 106, and waits for an acknowledgment from theneighbor 106 before sending another LSA protocol message to the neighbor106. Thus, the node 102 sends the first LSA protocol message 302 to theneighbor 106, and waits for an acknowledgment from the neighbor 106.Upon receiving the acknowledgment 304 from the neighbor 106, the node102 sends the second LSA protocol message 306 to the neighbor 106, andwaits for an acknowledgment from the neighbor 106. Upon receiving theacknowledgment 308 from the neighbor 106, the node 102 proceeds to sendsubsequent LSA protocol messages (not shown) in the same manner, untilall LSA protocol messages have been sent and acknowledged.

In an exemplary embodiment of the invention, the node 102 sends theseries of LSA protocol messages 200 to the neighbor 106 over thecommunciation link 104 using a sliding window mechanism. FIG. 4 is amessage flow diagram depicting an LSA protocol message exchange by alink state routing protocol using a sliding window mechanism using awindow size of eight (8). The node 102 is permitted to have up to eight(8) unacknowledged LSA protocol messages outstanding at any given time.Thus, the node 102 may send the first eight (8) LSA protocol messages(402, 404, 406, 408, 410, 412, 414, 416) to the neighbor 106 before thenode 102 must stop to wait for an acknowledgment from the neighbor 106for at least one of the LSA protocol messages. At this point, there areeight (8) unacknowledged LSA protocol messages outstanding, specificallythe LSA protocol messages (402, 404, 406, 408, 410, 412, 414, 416). FIG.5 depicts the position of the sliding window 502 after the node 102sends the first eight (8) LSA protocol messages to the neighbor 106.

Upon receiving an acknowledgment 418 for the first LSA protocol message402 from the neighbor 106, the node 102 sends the ninth LSA protocolmessage 420 to the neighbor 106. At this point, there are eight (8)unacknowledged LSA protocol messages outstanding, specifically the LSAprotocol messages (404, 406, 408, 410, 412, 414, 416, 420). FIG. 6depicts the position of the sliding window 602 after the node 102 sendsthe ninth LSA protocol message 420 to the neighbor 106.

Upon receiving an acknowledgment 422 for the second and third LSAprotocol messages (404, 406) from the neighbor 106, the node 102 sendsthe tenth and eleventh LSA protocol messages (424, 426) to the neighbor106. At this point, there are eight (8) unacknowledged LSA protocolmessages outstanding, specifically the LSA protocol messages (408, 410,412, 414, 416, 420, 424, 426). FIG. 7 depicts the position of thesliding window 702 after the node 102 sends the tenth and eleventh LSAprotocol messages (424, 426) to the neighbor 106.

In actuality, the neighbor 106 typically sends acknowledgments as itreceives and processes the LSA protocol messages. The neighbor 106 mayacknowledge each LSA protocol message individually, or, as with theacknowledgment 422 shown in FIG. 4, the neighbor 106 may acknowledgemultiple LSA protocol messages simultaneously. Thus, the node 102typically receives one or more acknowledgment messages from the neighbor106 before reaching the maximum number of outstanding unacknowledged LSAprotocol messages, and therefore the node 102 generally does not have tostop sending LSA protocol messages in order to wait for acknowledgmentmessages from the neighbor 106.

In the course of the LSA protocol message exchange between the node 102and the neighbor 106, it is possible for certain protocol messages to belost. This may be due to a communication error on the communication link104 or other reason. Protocol messages may be lost in either direction.For example, the neighbor 106 may not receive one or more LSA protocolmessages sent by the node 102, in which case the neighbor does notacknowledge the lost LSA protocol message(s). Also, the node 102 may notreceive one or more acknowledgment messages sent by the neighbor 106. Ineither case, the node 102 detects the error condition by failing toreceive an acknowledgment message for a particular LSA protocol messagewithin a predetermined timeout period. The node 102 preferablyretransmits unacknowledged LSA protocol messages after a predeterminedtimeout period. The node 102 may retransmit only specific unacknowledgedLSA protocol messages, although it is more typical for the node 102 toretransmit all LSA protocol messages from the unacknowledged LSAprotocol message onward. Thus, assuming the LSA protocol messages (408,410, 412, 414, 416, 420, 424, 426) are outstanding, as depicted by thesliding window 702 as shown in FIG. 7, and the LSA protocol message 408is unacknowledged, then the node 102 typically retransmits the LSAprotocol messages (408, 410, 412, 414, 416, 420, 424, 426). This is trueeven if the neighbor 106 received some or all of the LSA protocolmessages after the LSA protocol message 408.

In various embodiments of the invention, the sliding window mechanismmay be integrated with an existing link state routing protocol, such asOSPF, or-may be used in a new link state routing protocol. The presentinvention is in no way limited to a specific link state routingprotocol.

In an exemplary embodiment of the present invention, predominantly allof the routing protocol logic including the sliding window logic isimplemented as a set of computer program instructions that are stored ina computer readable medium and executed by an embedded microprocessorsystem within a node. Various embodiments of the invention may beimplemented in any conventional computer programming language. (e.g.,“C”) or an object oriented programming language (e.g., “C++”).Alternative embodiments of the invention may be implemented usingdiscrete components, integrated circuitry, programmable logic used inconjunction with a programmable logic device such as a FieldProgrammable Gate Array (FPGA) or microprocessor, or any other meansincluding any combination thereof.

Alternative embodiments of the invention may be implemented as acomputer program product for use with a computer system. Suchimplementation may include a series of computer instructions fixedeither on a tangible medium, such as a computer readable media (e.g., adiskette, CD-ROM, ROM, or fixed disk), or fixed in a computer datasignal embodied in a carrier wave that is transmittable to a computersystem via a modem or other interface device, such as a communicationsadapter connected to a network over a medium. The medium may be either atangible medium (e.g., optical or analog communications lines) or amedium implemented with wireless techniques (e.g., microwave, infraredor other transmission techniques). The series of computer instructionsembodies all or part of the functionality previously described hereinwith respect to the system. Those skilled in the art should appreciatethat such computer instructions can be written in a number ofprogramming languages for use with many computer architectures oroperating systems. Furthermore, such instructions may be stored in anymemory device, such as semiconductor, magnetic, optical or other memorydevices, and may be transmitted using any communications technology,such as optical, infrared, microwave, or other transmissiontechnologies. It is expected that such a computer program product may bedistributed as a removable medium with accompanying printed orelectronic documentation (e.g., shrink wrapped software), preloaded witha computer system (e.g., on system ROM or fixed disk), or distributedfrom a server or electronic bulletin board over the network (e.g., theInternet or World Wide Web).

The present invention may be embodied in other specific forms withoutdeparting from the essence or essential characteristics. The describedembodiments are to be considered in all respects only as illustrativeand not restrictive.

1. A method of distributing link state information by a node to aneighbor node in a communication network, the method comprisingimplementing a link state routing protocol having a sliding windowmechanism with a window size greater than one (1) for sending up to apredetermined maximum number of link state advertisement protocolmessages to the neighbor node without receiving an acknowledgement forany of said link state advertisement protocol messages.
 2. The method ofclaim 1, wherein implementing a link state protocol having a slidingwindow mechanism with a window size greater than one (1) for sending upto a predetermined maximum number of link state advertisement protocolmessages without receiving an acknowledgement for any of said link stateadvertisement protocol messages comprises: identifying link stateadvertisement messages that should be sent to the neighbor node; andmaintaining a sliding window for sending up to a predetermined maximumnumber of link state advertisement messages to the neighbor node withoutreceiving an acknowledgement message for any of said link stateadvertisement messages.
 3. The method of claim 1, wherein implementing alink state protocol having a sliding window mechanism with a window sizegreater than one (1) for sending up to a predetermined maximum number oflink state advertisement protocol messages without receiving anacknowledgement for any of said link state advertisement protocolmessages comprises: sending a first link state advertisement message tothe neighbor node; and sending a second link state advertisement messageto the neighbor node without receiving an acknowledgement message fromthe neighbor node for the first link state advertisement.
 4. The methodof claim 1, wherein implementing a link state protocol having a slidingwindow mechanism with a window size greater than one (1) for sending upto a predetermined maximum number of link state advertisement protocolmessages without receiving an acknowledgement for any of said link stateadvertisement protocol messages comprises: sending a first link stateadvertisement message to the neighbor node; and sending a second linkstate advertisement message to the neighbor node prior to receiving anacknowledgement message from the neighbor node for the first link stateadvertisement.
 5. The method of claim 3, comprising: monitoring for anacknowledgement from the neighbor node for the first link stateadvertisement; and retransmitting the first link state advertisementupon failing to receive an acknowledgement from the neighbor node forfirst link state advertisement within a predetermined time period. 6.The method of claim 5, comprising retransmitting the second link stateadvertisement.
 7. The method of claim 1, wherein implementing a linkstate routing protocol having a sliding window mechanism with a windowsize greater than one (1) for sending up to a predetermined maximumnumber of link state advertisement protocol messages without receivingan acknowledgement for any of said link state advertisement protocolmessages comprises: sending the predetermined maximum number of linkstate advertisements to a neighbor node; and waiting for anacknowledgement from the neighbor node for at least one of the linkstate advertisements before sending another link state advertisement tothe neighbor node.
 8. The method of claim 7, comprising: receiving anacknowledgement from the neighbor node; and sending another link stateadvertisement to the neighbor node after receiving the acknowledgement.9. The method of claim 7, comprising retransmitting at least a firstunacknowledged link state advertisement upon failing to receive anacknowledgement message from the neighbor node within a predeterminedtime period.
 10. The method of claim 1, comprising sending anacknowledgement from the neighbor node to the node in response toreceiving a link state advertisement from the node.
 11. A communicationnetwork, comprising a plurality of nodes, each operable to implement alink state routing protocol having a sliding window mechanism with awindow size greater than one (1) for sending up to a predeterminedmaximum number of link state advertisement protocol messages to aneighbor node without receiving an acknowledgement for any of said linkstate advertisement protocol messages.
 12. The network of claim 11,wherein each node is operable to implement a link state protocol havinga sliding window mechanism with a window size greater than one (1) forsending up to a predetermined maximum number of link state advertisementprotocol messages without receiving an acknowledgement for any of saidlink state advertisement protocol messages by: identifying link stateadvertisement messages that should be sent to the neighbor node; andmaintaining a sliding window for sending up to a predetermined maximumnumber of link state advertisement messages to the neighbour nodewithout receiving an acknowledgement message for any of said link stateadvertisement messages.
 13. The network of claim 11, wherein each nodeis operable to implement a link state protocol having a sliding windowmechanism with a window size greater than one (1) for sending up to apredetermined maximum number of link state advertisement protocolmessages without receiving an acknowledgement for any of said link stateadvertisement protocol messages by: sending a respective first linkstate advertisement message to the neighbor node; and sending arespective second link state advertisement message to the neighbor nodewithout receiving an acknowledgement message from the neighbor node forthe respective first link state advertisement.
 14. The network of claim11, wherein each node is operable to implement a link state protocolhaving a sliding window mechanism with a window size greater than one(1) for sending up to a predetermined maximum number of link stateadvertisement protocol messages without receiving an acknowledgement forany of said link state advertisement protocol messages by: sending arespective first link state advertisement message to the neighbor node;and sending a respective second link state advertisement message to theneighbor node prior to receiving an acknowledgement message from theneighbor node for the respective first link state advertisement.
 15. Thenetwork of claim 13, wherein each node is operable: to monitor for anacknowledgement from the neighbor node for the respective first linkstate advertisement; and to retransmit the first link stateadvertisement upon failing to receive an acknowledgement from theneighbor node for respective first link state advertisement within apredetermined time period.
 16. The network of claim 15, wherein eachnode is operable to retransmit the respective second link stateadvertisement.
 17. The network of claim 11, wherein each node isoperable to implement a link state routing protocol having a slidingwindow mechanism with a window size greater than one (1) for sending upto a predetermined maximum number of link state advertisement protocolmessages without receiving an acknowledgement for any of said link stateadvertisement protocol messages by: sending the predetermined maximumnumber of link state advertisements to a neighbor node; and waiting foran acknowledgement from the neighbor node for at least one of the linkstate advertisements before sending another link state advertisement tothe neighbor node.
 18. The network of claim 17, wherein each node isoperable: to receive an acknowledgement from the neighbor node; and tosend another link state advertisement to the neighbor node afterreceiving the acknowledgement.
 19. The network of claim 17, wherein eachnode is operable to retransmit at least a first unacknowledged linkstate advertisement upon failing to receive an acknowledgement messagefrom the neighbor node within a predetermined time period.
 20. Thenetwork of claim 11, wherein each node is operable to send anacknowledgement to a the neighbor node to the node in response toreceiving a link state advertisement from the neighbor node.